In the semiconductor industry, it is often necessary to create features on a substrate or other medium. A line and via are examples of a feature. Features are sometimes said to be created positively or negatively. By positively it is meant that the shape of the feature is carved out of its surrounding medium and the material comprising the feature is deposited in the carved out area of the medium.
Photoresists, which are light sensitive, are one way to create feature shapes. There are different types of photoresists. The choice of photoresists rests with the user and the desired critical dimensions. If the features to be created are close together or have an aspect ratio greater than 1, then a high resolution positive photoresist may be preferable. For the purposes of this discussion, the duration of time that it takes to effectively pattern a feature to the specification of the operator will be referred to as one patterning cycle. Historically, there has been one interval of exposure to a predetermined light intensity, through a mask, followed by one interval of baking followed by one development interval during one patterning cycle. Each time a photoresist is patterned using a mask, it is possible that the resulting feature shape will not look exactly the same as during a previous patterning cycle. It is also possible that the sidewalls of the feature will not have a consistent slope and/or profile, meaning that the shape of the features may not be optimal.
There is one attribute which is generally, universally, desirable in a high resolution positive photoresist, a feature with sidewalls that are substantially perpendicular to the bottom of the feature. Additionally, it is desirable that there be a minimal amount of scumming and residual in the bottom of the feature. As the number of features on a substrate and the complexity of the features increases, methods have been developed to ensure that the user specified critical dimensions can be consistently and accurately measured. One critical dimension that can be measured, as shown in FIG. 1, is the profile of the sidewall of the patterned resist. The feature, 1, has a left sidewall, 2, and a right sidewall, 3. Each sidewall has at least one slope, s. The measurement of the profile can provide information as to the extent of exposure. The information can be fed back and the dosage adjusted to ensure that the predetermined shape of the feature is achieved. If the profile of the feature is not constant, then the information fed back for dosage adjustment may be incorrect.